The NASA ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) mission is developing a fiber-laser, space-based LIDAR (Light Detection and Ranging) system for CO2 sensing. The CO2 absorption line centered at 1572.335 nm was chosen due to a confluence of several spectroscopic properties. The CO2 absorption line selected can be insensitive to temperature changes compared to other lines in the absorption band, free of absorption features from other atmospheric constituents, and have a convenient peak absorption amplitude that allows measurement of the full atmospheric column that optimizes the signal to noise ratio. The selected CO2 absorption line does not saturate, but is a large enough feature that it is easy to distinguish from background variations.
Fiber-based laser technology has a number of advantages for space-based LIDAR systems, such as efficiency, weight, and providing robust, alignment-free operation. However, there are some operational challenges. These challenges include that the measurement system requires low-repetition-rate (7.5 kHz), single-frequency, high-energy (>500 μJ) pulses at a wavelength that is longer than has been utilized for high-energy Er-doped fiber amplifiers. Long wavelength operation can require corresponding long amplifiers and narrow-linewidth, high-energy pulses that can result in stimulated Brillouin scattering (SBS). It can also be important for the system to keep polarization-maintaining operation and diffraction-limited beam quality.
Er-doped fiber based sources of high-energy, narrow linewidth pulses in the 15xx wavelength range for LIDAR applications have been used. However, they may work at wavelengths closer to 1550 nm, too short for CO2 detection, for example, 1.1 kW peak power at 1545 nm in a 108 ns pulse for a Yb-free Er fiber. Others may work at high pulse energies and peak powers, but the work is based on a multi-mode fiber and has poor M2. High aspect ratio, rectangular-core, Er-doped fibers produce very high pulse energies, but have not been demonstrated in an all-fiber format and the path to polarization maintaining operation is not clear. There is no polarization-maintaining demonstration for cladding-pumped, Yb-free Er fibers. A fiber laser for LIDAR using polarization-maintaining, commercial, off-the-shelf Er Yb fiber has a relatively small effective area of the core making peak power scaling difficult.
Very-large mode area, (VLMA) Er-doped fiber amplifiers, core pumped by highpower 1480 nm, Raman fiber lasers, generate diffraction limited, high energy pulses at 1.5 micron wavelengths, and have applications in femtosecond fiber chirp-pulse amplifiers and high-energy solution generation, for example, with core diameters greater than 50 microns and effective areas greater than 1100 μm2. However, polarization-maintaining amplifiers with the performance needed for the CO2 sensing application have not been demonstrated. Prior PM-VLMA fibers have suffered from difficulties with cleaving: the high stress used to increase birefringence to levels typical for PM fibers results in imperfections and surface distortion when the fibers are cleaved. This impairs fusion splicing, inhibiting robust all-fiber amplifier construction.
Polarization maintaining operation is important for many LIDAR systems, for example, the polarization extinction ratio was relatively poor in a multi-filament fiber with 37 Er Yb cores generated 940 W peak power with 1 MHz linewidth and an M2 of 1.3 at 1545 nm where the fiber had stress rods for polarization maintaining operation.